In recent years, there have been progressed developments of an image display device capable of transmitting a background as well as performing image display. For example, in Japanese Patent Application Laid-Open No. H5-191726, there is disclosed a presence display device that performs display by merging an image in a background. FIG. 30 is a view showing a configuration of a presence display device 200 disclosed in Japanese Patent Application Laid-Open No. H5-191726. As shown in FIG. 30, the presence display device 200 includes a projector 201, a transmission factor control screen 202, an image extracting device 203, and an image-outline forming device 204. The image extracting device 203 extracts an image to be displayed. The projector 201 projects the extracted image to the transmission factor control screen 202. The image-outline forming device 204 extracts outline information of the image to be displayed, and controls a state of the transmission factor control screen 202, based on the outline information. The transmission factor control screen 202 is specifically a PDLC (Polymer Dispersed Liquid Crystal) panel. The transmission factor control screen 202 is controlled such that only a region in which projection light from the projector 201 is applied becomes in a state of spreading light (hereinafter, referred to as a “spreading state”), and that other regions become in a state of allowing light to be transmitted (hereinafter, referred to as a “transmittance state”). With this configuration, the presence display device 200 can display, to an observer 205, the image to be displayed, by merging the image into the background including an ornamental tree 206 and a wall picture 207, for example.